The present invention relates to a method for the enrichment and purification of an aqueous solution of hydrogen peroxide. More particularly, the invention relates to an improvement in the method for the enrichment and purification of an aqueous solution of hydrogen peroxide in which a crude aqueous solution of hydrogen peroxide obtained, for example, by the anthraquinone method is subjected to evaporation in an evaporator into vapor and an accompanying liquid, the vapor is separated from the accompanying liquid in a gas-liquid separator and the vapor freed from the accompanying liquid is introduced into a fractionating distillation column to be enriched there into a high-purity enriched aqueous solution of hydrogen peroxide suitable for use in electronic industries or as a base solution of a super-high purity hydrogen peroxide solution required in the manufacture of semiconductor devices as well as for a reagent in a wide variety of chemical reactions.
As is known, hydrogen peroxide is produced currently by the so-called anthraquinone method involving the reaction of autoxidation of anthraquinone. In the anthraquinone method, namely, a 2-alkyl anthraquinone is hydrogenated in a water-insoluble organic solvent in the presence of a hydrogenation catalyst to give a corresponding anthrahydroquinone compound which is, after being freed from the catalyst by filtration, oxidized with oxygen or air to regenerate the starting anthraquinone compound with concurrent formation of hydrogen peroxide which is extracted with water to give an aqueous solution of hydrogen peroxide. The thus obtained aqueous solution of hydrogen peroxide contains considerable amounts of organic materials as impurities including the anthraquinone compounds and organic solvents as well as degradation products thereof so that it is a usual practice that the organic impurities in the aqueous hydrogen peroxide solution are removed by extraction with a water-immiscible organic solvent to give a primarily purified aqueous solution of hydrogen peroxide, referred to as a crude hydrogen peroxide solution hereinafter, containing decreased amounts of organic impurities. The content of hydrogen peroxide in a crude hydrogen peroxide solution is usually in the range from 15 to 40% by weight so that the solution in most cases must be enriched relative to the content of hydrogen peroxide, even by setting aside the problem relative to the content of impurities, since the aqueous hydrogen peroxide solution industrially required should have a content of hydrogen peroxide in the range from 30 to 70% by weight.
Various processes have been proposed as a method for the enrichment and purification of a crude hydrogen peroxide solution in U.S. Pat. No. 3,073,755, British Patent 1,326,282, Japanese Patent Publication 37-8256, Japanese Patent Publication 45-34926 and elsewhere. Each of these prior art processes utilizes, in principle, an apparatus system in which the crude hydrogen peroxide solution is evaporated in an evaporator into vapor with an accompanying liquid in the form of a mist which is separated and removed from the vapor in a gas-liquid separator and the vapor freed from the mist of liquid is introduced into a fractionating distillation column in which fractionating distillation is performed to give an enriched and purified aqueous hydrogen peroxide solution.
The aqueous hydrogen peroxide solution obtained by the above described prior art process can be used in most applications not only as a reagent for various chemical reactions but also as a bleaching agent, chemical polishing agent and the like. As a trend in recent years, the demand for aqueous hydrogen peroxide solutions is increasing year by year in the field of electronic industries including the manufacture of semiconductor devices and printed circuit boards. When an aqueous hydrogen peroxide solution is to be used in these applications in the electronic industries, it is essential that the aqueous hydrogen peroxide solution must have an extremely high purity relative to organic and inorganic impurities and the aqueous hydrogen peroxide solution prepared by the above described conventional process for the enrichment and purification of a crude hydrogen peroxide solution is not always satisfactory in this regard.
Namely, the above described conventional enrichment and purification processes of aqueous hydrogen peroxide solutions involve several problems when a high-quality aqueous hydrogen peroxide solution suitable for use in the electronic industries is desired. The crude hydrogen peroxide solution usually contains, besides organic impurities in a very low but non-negligible concentration, inorganic impurities originating in the surfaces of the apparatuses, pipe lines and the like. In addition, it is sometimes the case that the crude hydrogen peroxide solution contains stabilizers added to the reaction mixture in the manufacturing process with an object to prevent decomposition of hydrogen peroxide. Although these organic and inorganic impurities are not vaporizable, the vapor coming from the gas-liquid separator and introduced into the fractionating distillation column sometimes contains these impurities in the form of a mist of liquid accompanying the vapor due to incomplete gas-liquid separation in the gas-liquid separator so that the enriched and purified aqueous hydrogen peroxide solution obtained from the fractionating distillation column is necessarily contaminated with these impurities. Needless to say, various proposals and attempts have been made heretofore in order to improve the efficiency of the gas-liquid separation process using gas-liquid separators working on various principles including so-called demisters and the like.
Notwithstanding the proposals and attempts made heretofore, it is an extremely difficult matter to achieve perfect removal of the accompanying liquid in the form of extremely fine mist particles from the vapor produced from the crude hydrogen peroxide solution in an evaporator because the contacting surface area for the vapor in the gas-liquid separator cannot be large enough in consideration of the instability of hydrogen peroxide when contacted with the contacting surface of the gas-liquid separator. Despite the above mentioned problems relative to the efficiency of gas-liquid separation, no satisfactory process of gas-liquid separation and types or structures of gas-liquid separators as well as operating conditions thereof have yet been developed in order to meet the requirement for the purity of an enriched and purified aqueous hydrogen peroxide solution suitable for use in electronic industries.